I’ve been trying to get my head around Nuclear Fusion recently. Advocates suggest it will solve virtually every problem ever conceived if they ever get it up and running. I remain unconvinced, yet they still keep asking Governments all over the world for huge buckets of money, on the promise of limitless, ‘clean’, energy.
From the top then, for those who weren’t concentrating during nuclear physics at High School.
Atoms, the stuff all matter is made from, are made of a nucleus and orbitting electrons. In the nucleus there are always protons, and usually neutrons. The exception is Hydrogen, the simplest atom we know, which is often just one proton and an orbiting electron. The number of protons in an atom is what governs the type of atom it is; 1 proton for Hydrogen, 2 for Helium, 79 for Gold and 118 for the largest ever recorded atom, Ununoctium, of which 3 atoms have ever been recorded. This would have been manufactured as a part of some sort of heavy ion collision experiment. It would have been a short but spectacular life for an atom.
So, protons govern the type of atom, but also in the nucleus are neutrons, which don’t contribute much at all, except a bit of mass. Neutrons occur at about the same frequency as protons in a nucleus, but sometimes a couple more neutrons might be present. Hydrogen for example, can have none, one (deuterium) or two (tritium) neutrons in its nucleus. The mass number on the periodic table describes how frequently there are more neutrons. Mass number for hydrogen is 1.008, which means if you have a huge bucket of hydrogen atoms, their average mass will be 1.008 protons, the extra .008 describes atoms with the odd extra protons. 99.98% of all hydrogen atoms (and there are a LOT of them) are Hydrogen with one neutron though (H1).
Nuclear energy is all about the interaction of nuclei. There are 2 basic types of nuclear energy, fission (splitting atoms) or fusion (joining atoms together). Imagining watches as nuclei for a moment, which do you think is easier? Banging them together to see what comes out; or joining them together to make new things. Yep, smashing them open and taking the energy is heaps easier.
Fission is the ‘dirty’ nuclear that everyone knows. France has a huge nuclear industry, running 59 power plants. It produces massive amounts of energy and is pretty safe, considering how risky the concept is. There are ongoing concerns about how to store the waste though, which is one of the most poisonous substances known to man. Summarising; fission is a known quantity, it makes stacks of energy but the waste is a problem.
However, should we ever get a fusion industry going (and I strongly doubt it) there won’t be concerns about waste as the waste products are quite benign. Atoms are much more dangerous the larger their nucleus; bigger atoms can decay more easily which releases radiation. Fission uses huge uranium and plutonium atoms; fusion uses hydrogen and produces lithium.
The energy out of fusion reactions is extraordinary. Sadly, the best way to compare the 2 is with nuclear bombs. The most powerful fission bomb ever built is the equivalent to about 700,000 tonnes (700 kT) of TNT; some fusion bombs have been recorded at up to 15-50,000 kT of TNT. It’s a totally different ball park.
So, how does fusion work and why aren’t we using it everywhere? The fusion reaction is what powers our sun. There’s stacks of hydrogen in there, at incredible heat and pressure, with nuclei banging into each other continuously and emitting all of the energy we feel on earth. It’s good stuff. The lefty in me suggests that we could power our whole country from this fusion energy if we invested heavily in solar, but that’s another story. The real game in nuclear is creating and controlling the fusion reaction on earth.
It’s not really ‘controlled’ in the sun, which is why it works. You put enough hydrogen in one place and the gravitational pull of all that matter will squash it into the right conditions for fusion. It happens all over the universe in virtually every point of light you can see in the night sky.
The difficulty is in making a small, controllable, sun in a laboratory on Earth somewhere. This requires higher temperatures than the sun, because we can’t replicate the pressure in the sun as well. The goal of fusion is to bang hot nuclei together to make new nuclei, and harvest the energy that is created.
So, ideally it would work like this; a stack of hydrogen is ‘ignited’ to ‘plasma’ temperatures. Plasma is the 4th state of matter, in the same series as solids, liquids and gas. Imagine water; the solid is ice which occurs at zero degC. This melts and is a liquid from 0-100 degC, then over 100 it becomes a gas. In all cases the atoms are unchanged, but moving faster as the temperature goes up. But what if you wind the temperature up a lot, say from 100 degC to 100,000 degC. Now we’re getting near plasma temperatures.
Like liquid/gas phase change, the temperature (and pressure, they’re linked) that a substance becomes a plasma varies depending on the substance. In a liquid all the atoms are touching each other, but free to move around. A gas is when the atoms are no longer touching and moving around everywhere inside their vessel. Plasma though is once it is hot enough for the electrons to actually leave the atom and float around independently so creating a ‘soup’ of atomic particles; free electrons and nuclei all flying around together.
The ignition temperature can be achieved through a number of means; most commonly by applying a very strong magnetic field and recently using high-energy lasers. In isolation this is not very difficult, but keeping the reaction going is. What sort of material would you use to put a sun inside? Magnetic fields are the answer and current fusion experiments in a tokamak involve creating a plasma and containing it within a donut shaped magnetic field. In this way they can keep it moving around in circles without touching anything, which it would most certainly destroy.
Because the larger the atom is, the higher the energy required to cause collisions and start a plasma, most experiments are focussed on the lightest atoms; the hydrogen family. But to make the collisions more effective they concentrate on deuterium and tritium as the extra nuclei make the atoms a tiny bit bigger and heavier. So the reaction is run at temperatures to make hydrogen fuse; different atoms would require higher temperatures. And here is the problem.
As the reaction continues the hydrogen atoms involved in collisions become other things; Helium and Lithium in particular. The reaction is hot enough to fuse hydrogen, but nothing else, so as the heavier atoms are formed, it pollutes the reaction; in a very similar way to exhaust in a car. The moment the hydrogen atoms release their energy and become something else they go from helping the reaction to hindering it. To keep the reaction going, these new atoms need to be removed.
How on Earth can that be done? I doubt it can be, and I will eat a large hat if they crack it during our lifetime. Imagine a ‘gas’ cloud at 1 million degrees C. In it, spread randomly throughout are 2 different types of atoms. You need to keep one type in there and stinking hot; the other type needs to be removed because it’s wrecking the reaction.
If it is ever achieved, it will be achieved using oscillating magnetic fields which can exploit the minute mass difference of the 2 different atoms. I’ve been wondering if it is possible in a circulating fluid where there is no separation of reactants and pollutants; maybe they’ll end up with some sort of reciporocating device like a car engine, or a through-put device like a gas turbine. In either case it’s hard.
Despite all this, fusion research continues to tick along. At the moment the ITER guys are scouring the globe for forward looking governments willing to cough up for something that is unlikely to pay back during our lifetime. Current proposals require something like $50billion to build a new test bed reactor that they hope to have running by 2018. There are currently no designs or plans for demonstration reactors, let alone widespread grid connection. But like I said, if they crack it, a lot of problems will disappear.
Nerds all over the world live in hope, but personally, I’ve given up, and will concentrate on harnessing the fusion reaction that is already going and will be for a very long time to come.